1. Field of the Invention
The present invention relates to a rotation sensor mounted to a rotor used for detecting a rotation angle of the rotor.
2. Description of the Related Art
For example, when detecting the rotation angle of a handle mounted to a rotational shaft, such as a steering shaft of a motor vehicle, integrally therewith, so-called a rotation sensor is used.
As an example of such a rotation sensor, there is the one having stationary cores arranged so as to oppose to a rotor at a predetermined distance (For example, see JP-A-2003-202240).
This rotation sensor includes, as shown in FIG. 1 to FIG. 3 of the above-described patent publication, includes a rotor mounted to a rotating shaft, stationary cores each having a core body formed of insulative magnetic material and at least one exciting coil accommodated in the core body, and a rotation angle detecting unit. The exciting coil includes, for example, four exciting coils, which are arranged at regular intervals in the circumferential direction of the rotor, respectively.
The stationary cores are mounted to a fixed member positioned in the vicinity of the shaft, and are accommodated with the rotor in a case formed of metal or insulative magnetic material having a shielding property with respect to an alternating magnetic field, respectively.
The rotor includes rotor mounting portion formed of the insulative magnetic material and a sensing unit connected thereto via a stay member and continuously varying in width circumferentially thereof. The sensing unit is formed of conductive metal having a narrow portion having the minimum width and a wide portion having the maximum width located on the radially opposite side of the narrow portion, and is formed so that the width in the radial direction of the sensing unit varies according to the rotation angle of the rotor, whereby an eddy current having a magnitude corresponding to the width in association with the rotation is induced by the alternating magnetic field.
By using the rotation sensor configured as described above, the rotation angle between 0° and 360° of the rotor is detected by the use of variations in impedance of the exciting coil in association with generation of the eddy current.
Subsequently, referring to the drawing, an example of the rotation sensor relating to the invention will be described. As shown in FIG. 1, for example, there is a case in which four stationary cores 841a, 841b, 841c, 841d (831a, 831b, 831c, 831d) are arranged at the rotation angle of 90° in the sensor. By arranging the stationary cores 841a, 841b, 841c, 841d (831a, 831b, 831c, 831d) in this manner, the surface area of a circuit board 895 accommodated in a rotation sensor 801 is secured as large as possible.
More specifically, the stationary cores 831a, 841a, the stationary cores 831b, 841b, the stationary cores 831c, 841c, and the stationary cores 831d, 841d are mounted to a fixed member 890 which is located in the vicinity of the shaft via the respective coil core holders 892a, 892b, 892c, 892d, and accommodated with a rotor 810 in a case 820 formed of metal or insulative magnetic material having a shielding property with respect to an alternating magnetic field, respectively.
The rotor 810 includes a rotor mounting portion 811 formed of insulative magnetic material and a sensing unit 812 connected thereto via a stay member and continuously varying in width circumferentially thereof. The sensing unit 812 is formed of conductive metal having a narrow portion having the minimum width and a wide portion having the maximum width located on the radially opposite side of the narrow portion, and is formed so that the width in the radial direction of the sensing unit 812 varies according to the rotation angle of the rotor 810, whereby an eddy current having a magnitude corresponding to the width in association with the rotation is induced by the alternating magnetic field. Then, by using the rotation sensor 801 configured as described above, the rotation angle between 0° and 360° of the rotor 810 is detected by the use of variations in impedance of the exciting coil in association with generation of the eddy current.
The circuit block diagram of the rotation sensor as described above includes, as shown in FIG. 2, an oscillating unit 900 having an oscillation circuit 901 for outputting oscillation signals of a specific frequency, phase shifting unit 910 (911, 912, 913, 914) for shifting the phase of the oscillation signals supplied from the oscillation unit 900 according to the magnitude of the eddy current generated at the sensing unit, a phase shifting amount detecting unit 920 (921, 922, 923, 924) for detecting the phase shifting amount, a phase shifting amount convert unit 930 (931, 932, 933, 934) for converting the detected phase shifting amount to corresponding parameter, an amplifying unit 940 (941, 942, 943, 944) for amplifying the phase shifting amount outputted from the phase shifting amount converting unit 930, and a rotation angle detecting unit 950 for calculating the rotation angle based on the output from the amplifying unit 940, so that the rotation angle supplied to the phase shifting unit 910 is detected.
The phase shifting unit 910 includes a resistor, a capacitor, and a coil of the electronic circuit. The sensing unit of the rotor is continuously varied in width in the circumferential direction as described above, the impedance of the coil varies by rotation of the sensing unit of the rotor, which is interlocked with the rotation of the rotating shaft.
When the rotating shaft rotates, the output from the phase shifting amount detecting unit 920 with respect to the input angle is determined by the shape of the sensing unit, and hence it can be changed as a Sin waveform shown in FIG. 3. For example, as regards the two fixed cores (coil A, coil B) disposed at a center angle of 90° with respect to the center of the rotating shaft of the rotor, the phase shifting amount as a result of the signal processing based on the variation in impedance of the coil A of the stationary core on one side and the phase shifting amount as a result of the signal processing based on the variation in impedance of the coil B of the other stationary core vary with the phase difference of 90° with respect to the input angle as shown in FIG. 4. Although four stationary cores are shown in FIG. 1, the phase shifting amounts relating to two other stationary cores (coil C, coil D) are omitted in FIG. 4.
Then, with the rotation sensor in this arrangement, the rotation angle of the rotor is detected using the variations in impedance of the exciting coil in association with generation of the eddy current.
Subsequently, problems in achieving improvement of assembleability and improvement of detection accuracy of the rotation sensor described above will be described.
A first problem in achieving improvement of the detection accuracy of the aforementioned rotation sensor will be described. When mounting the rotation sensor as described above to a steering shaft of a motor vehicle, for example, and detecting the rotation angle of the steering shaft, there is a case in which a gap between the sensing unit of the rotor and the coil core varies due to vibrations of the vehicle, which results in errors in detection of output, and hence the rotation angle cannot be detected precisely.
In order to solve this problem, as shown in FIG. 13 to FIG. 16 in JP-A-2003-202240, there is a proposed rotation sensor having a structure in which four pairs of stationary cores in pairs are mounted to a case with the intermediary of the sensing unit of the rotor. The respective stationary cores in pairs each include a core body formed of insulative magnetic material and an exciting coil to be accommodated in the core body. Then the specific exciting coils are connected in series, and a magnetic circuit is formed around the fixed core by the AC exciting current from a measuring unit.
By disposing four pairs of stationary cores, each includes an upper stationary core and a lower stationary core, at a phase of 90° with the intermediary of the sensing unit of the rotor on one rotation sensor, variations in output due to variations in distances with respect to the respective stationary cores in the radial direction of the rotor caused by vibrations at the rotating portion is alleviated.
However, since the sensing unit of the rotor is required to be disposed between the upper and lower stationary cores of the respective pairs of the stationary cores in the rotation sensor as described above, the assembly process is inevitably divided for each pair of the upper and lower stationary cores from the limit of actual assembly of the rotation sensor.
The specific assembly process will be described as an example. As shown in FIG. 5, lower stationary cores 51–54 are assembled to a coil core holder 71, the assembled coil core holder 71 is assembled to a lower case 22, and a sensing unit 12 integrated with a rotor 10 in advance is assembled to the lower case 22.
On the other hand, as shown in FIG. 6, upper stationary cores 61–64 are assembled to a coil core holder 72, the coil core holder 72 is incorporated into the coil core holder 71, and the upper case (not shown) is fitted to the lower case 22 to complete a rotary connector.
In other words, for assembling the rotary connector in this process, the coil core holder 71 (72) for holding the respective stationary cores 51–54 (61–64) is divided into two parts of coil core holders 71, 72, to which totally four upper stationary cores 61–64 are mounted respectively as in the case of the four pairs in total of the lower stationary cores 51–54.
Therefore, the coaxiality of the respective pairs of the upper and lower stationary cores 51–54, 61–64 depends on the positional accuracy of the upper and lower coil core holders 71, 72, hence it is difficult to assemble the rotation sensor in an ideal dimensional relation due to the part tolerance or assembly tolerance. Consequently, in order to achieve an accurate assembly, a facility corresponding thereto must be used, which increases the cost significantly.
When an attempt is made to integrate the coil core holders 71, 72, which are formed separately, a sensing unit 12 of the rotor 10 has to be capable of being slid and inserted between the integrated coil core holders from the side. In other words, the relation of a>b is required between the dimension a and the dimension b shown in FIG. 5 and FIG. 6, whereby the dimension of the rotary sensor itself is also increased.
In contrast to the structure as described above, a structure of providing integrated four pairs of coil core holders 81–84 for retaining the upper and lower stationary cores 51–54, 61–64 independently as shown in FIG. 7 and FIG. 8 is also contemplated. However, with this structure, the relative position of the respective pairs, that is, the arrangement at positions shifted by 90° from each other with respect to the axis of a shaft S is difficult. It is because that the relative positions between the respective pairs of the stationary cores 51–54, 61–64 depend on the accuracy of the mounting positions of the respective coil core holders 81–84 with respect to the lower case 22.
In this manner, it is difficult to assemble the rotation sensor in a state in which the sensing unit 12 of the rotor 10 is interposed at suitable position between the respective pairs of the stationary cores, while arranging the stationary cores so as to oppose to each other precisely.
Also, in order to assemble the rotation sensor as described above, the number of stationary cores required is eight in total, which results in high cost. In order to achieve cost reduction in the rotation sensor, it is effective to reduce the number of the stationary cores. However, in order to reduce the number of stationary cores, it is required to prevent the output characteristic of the rotation sensor from being impaired.
When the structure in which the stationary cores 51–54, 61–64 are disposed at four positions of the rotary sensor as described above is employed, there are various limits in assembly or component structure caused by necessity of the sensing unit 12 of the rotor 10 to be arranged between the opposed stationary cores, and a number of the stationary cores must be used for one rotation sensor, which is an obstacle for providing a cost-effective rotation sensor with high degree of accuracy.
Subsequently, a second problem in achieving improvement of the detection accuracy of the aforementioned rotation sensor will be described.
The rotation sensor 801 in the related art is configured as shown in FIG. 1, and is mounted to a steering shaft S of a vehicle via a sensor mounting member 300 (see FIG. 11). FIG. 9 shows a plan view of the rotation sensor 801 relating to the invention, and FIG. 10 shows a side view of the rotation sensor 801 relating to the invention. As is seen from FIG. 9 and FIG. 10, the case 820 includes an upper case 821 and a lower case 822, and the outer peripheral portion of the lower case 822 is fitted to a mounting rib 303 (see FIG. 11) of the sensor mounting member 300 described later. The lower case 822 is formed with an engagement projection 825 projecting therefrom. The engagement projection 825 for mounting the rotation sensor 801 to the sensor mounting member 300 is formed so as to project from the rotation sensor 801 at a circumferentially predetermined position slightly shifted rightward in the drawing when viewed in the longitudinal direction as shown in FIG. 9. By engaging the engagement projection 825 with an engaging notch 305 of the sensor mounting member 300, the rotation sensor 801 is mounted to the sensor mounting member 300. The lower case 822 is provided with a connector unit 826 for electrically connecting the detection circuit of the rotation sensor 801 and the external wire harness.
On the other hand, the sensor mounting member 300 includes, as shown in FIG. 11, a shaft insertion hole 301 for inserting the steering shaft S at the central portion thereof, and an abutting portion 302 which abuts against the case 820 of the rotation sensor 801 on the periphery thereof, and the abutting portion 302 is formed with the sensor retaining rib 303 on the outer periphery thereof. Part of the abutting portion 302 and the sensor retaining rib 303 are formed with the engagement notch 305 so as to engage with the engagement projection 825 provided with the rotation sensor 801. The sensor mounting member 300 is provided with a bracket, not shown, so as to fix the same to the vehicle.
Then, by passing the steering shaft S through the center portion of the rotation sensor 801 and fitting the outer periphery of the case of the rotation sensor 801 to the retaining rib 303 of the sensor mounting member 300 while engaging the engagement projection 825 of the rotation sensor 801 with the engagement notch 305 of the sensor mounting member 300, so that the rotation sensor 801 is mounted to the sensor mounting member 300.
When mounting the rotation sensor 801 to the sensor mounting member 300, in order to improve the detecting characteristic of the rotation sensor 801, the engagement projection 825 of the rotation sensor 801 and the engagement notch 305 of the sensor mounting member 300 are formed so that the rotation sensor 801 is mounted so as not to rattle in the circumferential direction in a state of being mounted and simultaneously so as to generate rattling to a certain extent in the radial direction in a state of being mounted in order to facilitate mounting of the rotation sensor 801 to the sensor mounting member 300.
Accordingly, the rotor 810 of the rotation sensor 801 is rotatably fixed together with the steering shaft S, and the case 820 of the rotation sensor 801 and the stationary cores 831a, 831b, 841a, 841b shown in FIG. 1 are mounted to the sensor mounting member 300 with rattling radially of the sensor to a certain extent.
The engagement projection 825 of the rotation sensor 801 is formed at a position shown in FIG. 9 as described above is in the case in which there are various constrains in mounting dimension with respect to the engagement notch 305 formed on the sensor mounting member 300 or the like.
The stationary cores 831a, 831b, 841a, 841b provided on the rotation sensor 801 are disposed at positions as shown in FIG. 1 at rotation angles of 90° from each other, so that the surface area of a circuit board 895 in the rotation sensor remains as large as possible as described above.
When the engagement projection 825 of the rotation sensor 801 (see FIG. 9 and FIG. 10), the stationary cores 831a, 841a (coil A) and the stationary cores 831b, 841b (coil B) are in such a positional relation, the stationary cores 831b, 841b on the right side in FIG. 1 are positioned in the vicinity of the engagement projection 825 with respect to the rotation sensor 801, while the stationary cores 831a, 841a on the left side in the figure are positioned at a position significantly apart form the engagement projection 825 of the rotation sensor 801.
FIG. 12 is a characteristic drawing showing a relation between an angle formed between a connecting line (mounting reference line) between the center axis of the steering shaft S and the engagement projection 825 and the connecting portion between the steering shaft S and the stationary cores 831a, 831b, 841a, 841b shown on the lateral axis and the degree of influence of circumferential displacement of the rotation sensor 801 generated by displacement of the engagement projection 825 radially of the sensor shown in the vertical axis.
As will be understood from FIG. 12, since the stationary cores 831a, 841a (corresponding to the coil A in FIG. 12) on the left side in FIG. 1 is significantly displaced in the circumferential direction with respect to the connecting line between the axis of the steering shaft S and the engagement projection 825, radial displacement of the rotation sensor 801 gives significant influence to circumferential displacement of the rotation sensor 801. When such an influence is significant even on one of the stationary cores, the detection characteristic of the entire rotation sensor is adversely affected.
Subsequently, a third problem in achieving improvement of the detection accuracy of the aforementioned rotation sensor will be described.
In the rotation sensor in the related art, the sensing unit of the rotor is fixed to the rotating shaft, and the exciting coil is fixed to the case via the stationary core. In other words, when mounting the rotation sensor, the rotor side of the rotation sensor is mounted to the rotating shaft, and the stator side is mounted to the portion other than the rotating shaft via a bracket or the like. Therefore, there may arise a displacement between the sensing unit of the rotor and the exciting coil to a certain extent in order to achieve improvement of the mounting property of the rotation sensor. It causes no problem when the displacement is within a tolerance. However, when the displacement exceeds the tolerance, an unallowable displacement is generated in phase shifting amount with respect to the input angle as shown in FIG. 13 due to rattling between the sensing unit of the rotor and the exciting coil and the temperature characteristic of the coil or the like. In FIG. 13, a case in which the phase shifting amount is displaced (displaced vertically in the graph) is shown. However, there may be a case in which the input angle is displaced (displaced laterally in the graph), and displacement in both directions may occur. Actually, between the sensing unit of the rotor and the exciting coil, displacement in the radial direction of the sensing unit may easily occur. At this time, in a rotation angle detecting unit 950 (see FIG. 2), the angle which is displaced from the actual input angle is detected as is. However, only with the present structure, it is difficult to detect occurrence of such a displacement which is acceptable. Supposing that an attempt is made to detect such displacement, additional parts are required, which results in increase in cost. In this manner, since it is difficult to discriminate variation in detection output caused by rattling between the sensing unit of the rotor and the exciting coil or the temperature from essential variations in detecting angle, when unexpected displacement occurs, it cannot be diagnosed as an abnormal state, and hence the rotation angle may be erroneously detected. When the unallowable positional displacement between the sensing unit of the rotation sensor and the exciting coil as described above can be determined, a countermeasure such as canceling the sensor output signal can be taken as needed. However, as long as the positional displacement as such cannot be determined, it is difficult to take a suitable countermeasure.